Process for producing phosphorous acid

Abstract

Phosphorous acid is made by a continuous recycling process from a mixture of phosphorous and hydrochloric acids, comprising: (i) continuously recycling the mixture in a loop apparatus; (ii) adding water to and passing phosphorus trichloride into said recycling mixture so as to generate a condition of turbulent flow at the point of contact of the phosphorus trichloride and the recycling mixture, whereby a reaction product, comprising an aqueous solution of phosphorous and hydrochloric acids as a continuous liquid phase and gaseous hydrogen chloride as gas bubbles of disperse phase, is produced; (iii) separating said gaseous hydrogen chloride; (iv) cooling said reaction product; and (v) recycling at least a portion of said reaction product and repeating steps (ii), (iii) and (iv) thereon to produce a more highly concentrated solution of phosphorus acid.

We claim: 
     
       1. A process for the production of phosphorous acid by the reaction of phosphorus trichloride with a mixture of phosphorous acid and hydrochloric acid, said process consisting of the following steps: (i) continuously recycling the mixture in a loop apparatus at a temperature in the range of 50° C. to 180° C.;   (ii) adding water to and passing phosphorus trichloride into said recycling mixture so as to generate a condition of turbulent flow at the point of contact of the phosphorus trichloride and the recycling mixture, whereby a reaction product, comprising an aqueous solution of phosphorous and hydrochloric acids as a continuous liquid phase and gaseous hydrogen chloride as gas bubbles of disperse phase, is produced, the amount of water added is sufficient to hydrolyze the phosphorus trichloride;   (iii) separating said gaseous hydrogen chloride;   (iv) cooling said reaction product; and   (v) recycling at least a portion of said reaction product and repeating steps (ii), (iii) and (iv) thereon to produce a more highly concentrated solution of phosphorus acid.   
     
     
       2. The process of claim 1, wherein said loop apparatus comprises separate inlets for said water and said phosphorus trichloride, together with a gas separator, at least one mixer, cooling means and at least one pump. 
     
     
       3. The process of claim 1, wherein said loop apparatus includes two or three mixers. 
     
     
       4. The process of claim 3, wherein at least one of said two or three mixers is a static mixer having no moving parts. 
     
     
       5. The process of claim 4, wherein said mixer is a turbulent-flow orifice plate mixer having an orifice substantially perpendicular to said plate and a port in the internal wall of said orifice, whereby said recycling liquid mixture flows through said orifice and phosphorus trichloride passes through said port. 
     
     
       6. The process of claim 5, wherein said orifice has a ratio of diameter to length which is in the range 0.1:1 to 50:1. 
     
     
       7. The process of claim 6, wherein ratio is in the range of 0.25:1 to 20:1. 
     
     
       8. The process of claim 6, wherein said ratio is in the range 0.25:1 to 10:1. 
     
     
       9. The process of claim 1, wherein said water and said phosphorus trichloride are passed continuously into said loop apparatus and said reaction product is removed continuously therefrom. 
     
     
       10. The process of claim 1, wherein said temperature is in the range 50° C. and 90° C. 
     
     
       11. The process of claim 1, wherein said concentration of phosphorous acid in said recycling liquid mixture is from 50% to 99%. 
     
     
       12. The process of claim 11, wherein said concentration is from 70% to 90%. 
     
     
       13. The process of claim 1, wherein the amount of water added to said recycling liquid mixture is from 3 and 9 moles of water per mole of phosphorus trichloride. 
     
     
       14. The process of claim 13, wherein said amount of water is from 3 to 6 moles of water per mole of phosphorus trichloride. 
     
     
       15. The process of claims 9, wherein the ratio of said recycled reaction product to said removed reaction product is in the range 20:1 to 2000:1. 
     
     
       16. The process of claim 15, wherein said ratio is in the range 30:1 to 200:1. 
     
     
       17. The process of claim 15, wherein said ratio is in the range 40:1 to 150:1. 
     
     
       18. The process of claim 11, wherein the amount of water added to said recycling liquid mixture is from 3 and 9 moles of water per mole of phosphorus trichloride; and the ratio of said recycled reaction product to said removed reaction product is in the range 20:1 to 2000:1. 
     
     
       19. The process of claim 18, wherein said temperature is in the range of 50° C. and 90° C.;   said concentration is from 70% to 90%;   said amount of water is from 3 to 6 moles of water per mole of phosphorus trichloride; and   said ratio is in the range 40:1 to 150:1.   
     
     
       20. The process of claim 18, wherein said loop apparatus comprises separate inlets for said water and said phosphorus trichloride, together with a gas separator, a static mixer having no moving parts, cooling means and at least one pump;   said mixer is a turbulent-flow orifice plate mixer having an orifice substantially perpendicular to said plate and a port in the internal wall of said orifice, whereby said recycling liquid mixture flows through said orifice and phosphorus trichloride passes through said port;   said orifice has a ratio of diameter to length which is in the range 0.1:1 to 50:1; and   said water and said phosphorus trichloride are passed continuously into said loop apparatus and said reaction product is removed continuously therefrom.   
     
     
       21. The process of claim 20, wherein said ratio of orifice diameter to length is in the range 0.25:1 to 10:1.